Method for draining a delivery unit for liquid additive, delivery unit and motor vehicle having a delivery unit

ABSTRACT

A method for draining a delivery unit for a liquid additive, in particular a urea-water solution, includes at least: a) producing an electrical connection by using a currentless switch which is constructed to produce an electrical connection when a temperature falls below a threshold temperature; and b) draining the delivery unit when, in step a), the electrical connection has been produced. The method can be used, in particular, in SCR systems for exhaust-gas aftertreatment for motor vehicles. A delivery unit and a motor vehicle having a delivery unit are also provided.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation, under 35 U.S.C. §120, of copending International Application No. PCT/EP2013/057896, filed Apr. 16, 2013, which designated the United States; this application also claims the priority, under 35 U.S.C. §119, of German Patent Application DE 10 2012 103 453.2, filed Apr. 19, 2012; the prior applications are herewith incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to a method for draining a delivery unit for liquid additive. The invention also relates to a delivery unit and a motor vehicle having a delivery unit.

Such delivery units are used, for example, for delivering urea-water solution as liquid additive into the exhaust-gas treatment device of an internal combustion engine. Urea-water solution is used in exhaust-gas treatment devices for the removal of nitrogen oxide compounds from the exhaust gases within the context of the SCR process [SCR=selective catalytic reduction]. A 32.5% urea-water solution which is available under the trademark AdBlue® is typically used.

A problem with such delivery units is the fact that the liquid additives can freeze during long standstill periods of the delivery units. The described urea-water solution, for example, freezes at −11° C. Such low temperatures may arise, in particular, during long standstill phases of a motor vehicle in winter.

Water-based liquid additives, in particular, (such as the urea-water solution) expand in terms of their volume as they freeze. That can damage the delivery unit. It is therefore known from the prior art to drain a delivery unit upon the cessation of operation of a motor vehicle. The liquid additive which is located in the delivery unit is usually delivered back, during the draining process, into a tank in which the liquid additive is stored.

It is a problem in that case that the draining of the delivery unit typically leads to a cumbersome configuration of the system and/or to increased consumption of the additive. Increased consumption occurs, for example, because the liquid additive located in the delivery unit cannot be completely recovered when draining occurs. Under certain circumstances, a residual quantity of the liquid additive remains in the delivery line. That residual quantity can evaporate there. As a result of the draining, it is also necessary to refill the delivery unit when re-activation occurs. During that refilling it is also possible for a loss of liquid additive to occur, for example because when refilling occurs an excess of liquid additive occurs and the excess escapes from a feed device of the delivery unit.

Under certain circumstances, delivery unit wear which occurs as a result of the draining process is also problematic. In particular, a pump of the delivery unit can be subject to increased wear as a result of the draining process.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a method for draining a delivery unit for liquid additive, a delivery unit and a motor vehicle having a delivery unit, which overcome the hereinafore-mentioned disadvantages and solve or at least alleviate the highlighted technical problems of the heretofore-known methods, units and vehicles of this general type. It is sought, in particular, to disclose an especially suitable method for draining a delivery unit. It is likewise sought to specify a delivery unit for realizing the method which is simply constructed and, if appropriate, can also be retrofitted. In particular, the method and/or the delivery unit should be suitable for use in SCR exhaust-gas aftertreatment systems in the automotive field.

With the foregoing and other objects in view there is provided, in accordance with the invention, a method for draining a delivery unit for liquid additive, comprising at least the following steps:

-   a) producing an electrical connection by using a currentless switch     which is constructed to produce an electrical connection when a     temperature falls below a threshold temperature; and -   b) draining the delivery unit when, in step a), the electrical     connection has been produced.

The delivery unit that can be drained by using the described method preferably has at least one delivery line which extends from a tank for the liquid additive to a feed device (nozzle, injector, valve or the like) for the feed of the liquid additive to a consumer (for example an exhaust-gas treatment device). The delivery line is connected to the tank at an extraction point so that liquid additive from the tank can enter the delivery line. A pump, which is preferably disposed in the delivery line, pumps the liquid additive through the delivery line. The delivery line and the pump are filled with liquid additive during the delivery operation of the delivery unit. The delivery unit may have further components filled with liquid additive.

The method is implemented specifically when the delivery unit and/or the motor vehicle are/is deactivated, that is to say, for example, no ignition is applied or the engine is inactive. The method may thus be activated, for example, when the motor vehicle is deactivated.

The switch used in step a) is preferably distinguished in that the operation thereof does not require any current before the electrical connection has been produced. It is preferable if, in the switch, a mechanical and/or physical effect is utilized which produces the electrical connection. The thermal expansion of a material or phase changes of a material may be specified as examples for correspondingly utilizable mechanical and/or physical effects. The switch is thus in a currentless standby position before step a). The switch is referred to in step a) as a currentless switch. This also means, in particular, that the switch is not actuated by an electrical connection. There is, in particular, no electrical actuation line through which an electrical signal can arrive at the switch in order to control the position of the switch.

The switch is preferably configured in such a way that it produces the electrical connection when a temperature in the surroundings of the switch reaches or drops below a (lower) threshold temperature. This threshold temperature is typically above a freezing temperature of the liquid additive being used. The threshold temperature is determined by the structure of the switch.

Basically, the switch can interact with the delivery unit and/or is part of the delivery unit itself. If the delivery unit is integrated into a motor vehicle, the switch can, where appropriate, then also be provided at a position which is disposed remotely from the delivery unit. The position of the switch should be selected, in particular, in such a way that with the switch it is possible to acquire or predict information which is characteristic of the current aggregate state of the additive. For this purpose, the delivery unit has, in particular, a detection device with which the current state of the switch can be detected, in particular it can be detected whether the latter is currentless or energized. Of course, it may also be appropriate to provide a plurality of such switches which can then make available information on the ambient temperature and/or the aggregate state of the additive for, for example, different sections of the delivery unit. The information which is acquired with a plurality of switches can be used to define when partial/complete draining is to be initiated or carried out.

At least a (significant) part of the liquid additive, but preferably all of the liquid additive, is transported (in particular in the pump and/or the delivery line) out of the delivery unit components which are filled with liquid additive, during the draining process in step b). It is expedient for such an amount of liquid additive to be transported out that the residual amount of liquid additive that may remain in the delivery unit cannot lead to damage to the delivery unit if the liquid additive freezes.

Due to the described method, it is not necessary to perform a draining during every operational stoppage of the delivery unit or of the motor vehicle. It is rather also possible for a draining to be performed as required when there is a threat of freezing of the liquid additive due to low outside temperatures. For example, urea-water solution with a 32.5 percent urea content freezes at −11° C. It is therefore advantageous if the threshold temperature of the switch is set, for example, in the range from −7° C. to −10° C. in order to drain the delivery unit when there is a threat of freezing of the liquid additive.

No permanent current supply (of the dedicated and/or superordinate controller) is required for the described method while the motor vehicle is at a standstill. Due to the described method, there is no (significant) current consumption for as long as draining is not being performed. The switch is therefore used to “wake” the components required for the draining only when the predefined threshold temperature is attained outside or in a temperature-sensitive region of the delivery unit.

As a result of the described method it becomes possible in particular, also to avoid the sucking of impurities into the delivery line of the delivery unit. During the draining, the liquid additive is preferably delivered back into the tank from the delivery unit counter to the delivery direction. Within the process, the liquid additive is replaced by gas or by air. The gas or the air is preferably sucked in by a supply device during the draining process. If the supply device is disposed at an exhaust-gas treatment device, under certain circumstances impurities from the exhaust gas are also sucked into the delivery unit. As a result of the fact that the draining is not carried out whenever a motor vehicle is deactivated but rather only when the present temperature actually requires it, the risk of contamination of the delivery line by impurities from the exhaust gas can be considerably reduced.

In accordance with another particularly preferred mode of the method of the invention, a control unit is activated by using the switch in step a), and the control unit controls the draining in step b).

It is preferable if a control unit is activated by using the switch in step a) and the control unit controls the draining process. In order to carry out the draining process it is, for example, the case that a pump is activated for the draining. It may additionally be expedient for valves to be opened, through which the draining then takes place. In order to coordinate the actuation of the pump and if appropriate the actuation of valves, it is advantageous for a control unit to be used which performs the coordination. The control unit can, for example, also be the engine control unit of a motor vehicle which is activated or woken up by step a).

In accordance with a further advantageous mode of the method of the invention, the control unit automatically deactivates after the execution of the method.

The current consumption of the method can be further reduced by using an automatic deactivation of the control unit after the execution of step b). For the purpose of deactivation, for example a deactivation switch is present which, like the currentless switch, is configured to interrupt the electrical connection. In contrast to the currentless switch, the deactivation switch is not actuated in a currentless fashion as a function of the temperature but instead can be actuated by the control unit itself through a signal line. The deactivation switch is actuated only if the delivery line is already drained (to the desired extent). If the deactivation switch has been activated, it is no longer possible to activate the control unit by using the currentless switch. The deactivation switch can also be integrated in the control unit. In other words, the control unit can have an (internal) shut-down device which deactivates the control unit independently of the position of the currentless switch.

In accordance with an added advantageous mode of the method of the invention, between step a) and step b), a temperature measurement is performed by using a temperature sensor in order to check whether or not a threshold temperature has actually been undershot, and the liquid additive is removed from the delivery unit in step b) only if the threshold temperature has actually been undershot.

In order to avoid a fault as a result of an erroneous tripping of the currentless switch in step a), it is advantageous if, before the initiation of step b), the control unit additionally performs a check of the temperature by using (at least) an (additional and/or electrical) temperature sensor, in order to ensure that the delivery unit is drained only if the threshold temperature has actually been undershot. The (sensor-measured) threshold temperature may deviate from the threshold temperature for the activation of the currentless switch in step a), in order, if appropriate, to avoid an unnecessary draining of the delivery unit in step b). For example, a threshold temperature of the switch can be set in such a way that the control unit is already activated at a temperature of −7° C. If the temperature drops to −10° C., the actual draining (step b) can then be carried out, with this then being triggered by a measurement result determined at the temperature sensor. Due to such additional checking of the temperature it is possible to detect particularly reliably situations in which draining of the delivery line is necessary. In particular it is also possible for the accuracy of the limiting temperature of the currentless switch to be lower than the accuracy of the temperature measurement with the additional temperature sensor. For example, the threshold temperature can have an uncertainty level of more than ±3° C., while the accuracy of the additional sensor is less than ±1° C. As a result of the additional checking, this inaccuracy of the currentless switch can be at least partially compensated.

In accordance with an additional advantageous mode of the method of the invention, after step b), it is stored in a memory that the delivery unit has been drained, and before step a) and/or after step a), it is checked on the basis of the information in the memory whether or not the system has already been drained, and at least step b) is carried out only when the system has not been drained.

By virtue of the fact that the present state of the delivery unit (drained or not drained) is stored in a memory, it is possible to avoid situations in which the described method is executed multiple times during a (single) stoppage of operation. It is possible, in particular, to avoid a situation in which the delivery unit is damaged by the attempt to re-drain the already drained delivery unit. An (electronic) memory for checking whether step b) is to be carried out can alternatively or additionally be taken into account for the deactivation of the control unit with a deactivation switch. The information as to whether draining was carried out or not, stored in the memory, can, in particular also be used during re-activation of the delivery unit in order, where appropriate, to perform refilling of the delivery unit when the delivery unit has been drained.

In accordance with yet another advantageous mode of the method of the invention, the delivery unit is drained by back-suction of the liquid additive back into a tank.

Back-suction counter to a normal delivery direction is a particularly advantageous way of draining the delivery unit in step b). The back-suction is made possible by using a (single) pump which can be operated counter to the normal delivery direction. In a further structural variant, it is also possible to provide a return line which branches off from the delivery line, in such a way that draining is performed by using delivery in a circuit. It is also possible for a separate pump to be used for the back-suction.

Draining preferably takes place counter to a (normal) delivery direction of the delivery unit, wherein the (normal) delivery direction extends from the tank to the supply device along a delivery line. The delivery direction of the pump is reversed for this purpose. Depending on the construction of the pump, this can take place by reversing the drive direction of the pump or by suitable switching of valves which are assigned to the pump.

In particular, during the back-suction, there is the risk of air being sucked into the delivery unit through a feed device (injector, etc.). The feed device is the same feed device used for feeding the liquid additive to a consumer (for example an exhaust-gas treatment device). It is also possible for an additional valve to be provided through which air can be sucked into the delivery unit during the draining.

A further possibility for the draining in step b) is for the delivery unit to be (partially) blown empty by using air. For this purpose, a compressed-air line may be utilized which is connected to the delivery unit or to the line system.

In accordance with yet a further advantageous mode of the method of the invention, the switch is at least a switch from the following group:

-   a bimetal switch, -   a switch including a shape memory alloy, -   a temperature switch with a liquid, and -   a prestressed switch.

In the case of a bimetal switch, two different materials with different thermal coefficients of expansion are disposed adjacent one another and connected to one another. The form/shape of the bimetal configuration varies as a result of temperature variations. For example, the configuration bends. The variation of the form/shape may be utilized to mechanically close a contact. An electrical connection can be closed in this way.

In the case of a shape memory alloy, the form of a component varies due to a phase change, whereby the electrical connection can be closed. A nickel-titanium alloy is mentioned herein as an example of a shape memory alloy. Polymer materials which exhibit shape memory characteristics may also be used for the switch.

A temperature switch with a liquid may, for example, have a sleeve filled with liquid. If the liquid freezes, its volume changes. The change in volume can be utilized to thereby close an electrical connection.

A (mechanically) prestressed switch which is used for the described method is preferably constructed in such a way that when the threshold temperature is reached the switch experiences a sudden change in shape which produces an electrical contact. Such a switch can include, for example, a prestressed expansion material (for example a mechanically prestressed metal) which up to the threshold temperature is located in a first position and changes into a second position when the threshold temperature is reached. In this context, in the first position the expansion material is preferably located in a prestressed (unstable) position and is prestressed (for example against a stop). When the threshold temperature is exceeded, the expansion material can no longer be held in the first position and changes (suddenly) into a second position. Where appropriate, an actuator with which the prestressed switch can be actively moved back into the prestressed first position can also be present.

In accordance with yet an added advantageous mode of the method of the invention, the method is executed during a standstill phase of an internal combustion engine. In this context it is also particularly advantageous if the draining or the execution of step b) is (actively) prevented during the operation of an internal combustion engine.

During an operating phase of an internal combustion engine, the delivery unit is usually required to feed the liquid additive to the exhaust-gas treatment device of the internal combustion engine in order to ensure effective exhaust gas purification. The draining should therefore preferably not take place. It is therefore advantageous if the non-electronic switch is bypassed and/or blocked during operation of an internal combustion engine.

With the objects of the invention in view, there is also provided a delivery unit comprising a currentless switch such as is used for step a) of the described method. The advantages and structural features explained for the described method can be transferred analogously to the described delivery unit. The delivery unit preferably has a housing in which the pump, the switch and at least one portion of the delivery line are situated. In the housing there may additionally be provided a control unit or controller which is configured or programmed to execute the described method. The delivery unit with the switch constitutes a component that can be installed in a motor vehicle (in particular a tank).

With the objects of the invention in view, there is concomitantly provided a motor vehicle which will be described by way of example in the text which follows.

The motor vehicle is shut down at temperatures above the freezing point of urea, wherein the freezing point of urea is the freezing point of the liquid additive. The control unit is in a rest state (ignition off). If the vehicle is shut down for a relatively long period of time, it must be ensured that repeated draining by suction (activation of pump and injector) does not take place. Draining by suction can be performed only once between two engine starting processes. The following measures may therefore be implemented:

-   a) by using a bimetal switch (or some other mechanically tripped     contact/switch) which switches a contact at approximately −9° C.,     the control unit is activated, and the injector is opened and the     system is drained by suction. The information that draining by     suction has taken place is stored and is accessed upon a restart of     the system, in such a way that the system knows that draining by     suction has taken place; -   b) without the control unit, the pump and the injector are     activated, and the action is stored in a type of memory, in such a     way that the information can be accessed upon a start of the engine.

In this way, draining of the system as required is ensured, and the back-suction takes place only when the exhaust system has cooled down and thus also the risk of the ingestion of particles has been minimized.

Other features which are considered as characteristic for the invention are set forth in the appended claims, noting that the features specified individually in the claims may be combined with one another in any desired technologically meaningful way and may be supplemented by explanatory facts from the description.

Although the invention is illustrated and described herein as embodied in a method for draining a delivery unit for liquid additive, a delivery unit and a motor vehicle, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a schematic and block diagram of a motor vehicle having a delivery unit which is configured to carry out the method described herein;

FIG. 2 is a diagrammatic, longitudinal-sectional view of a first embodiment variant of a currentless switch;

FIG. 3 is a longitudinal-sectional view of a second embodiment variant of a currentless switch; and

FIG. 4 is a longitudinal-sectional view of a third embodiment variant of a currentless switch.

DETAILED DESCRIPTION OF THE INVENTION

Referring now in detail to the figures of the drawings, in which illustrated size ratios are only diagrammatic, and first, particularly, to FIG. 1 thereof, there is seen a motor vehicle 11, having at least one internal combustion engine 8 and having an exhaust-gas treatment device 12 for the purification of exhaust gases of the internal combustion engine 8. The exhaust gases from the internal combustion engine 8 can be purified with an SCR catalytic converter 14 provided in the exhaust-gas treatment device 12. The method of selective catalytic reduction can be carried out at the SCR catalytic converter 14 using liquid additive (such as an aqueous urea solution), which can be fed to the exhaust-gas treatment device 12 with a feed device 13 on a delivery unit 1. The feed device 13 has a pump 10 and a delivery line 9. The delivery line 9 runs from an extraction point 17 on a tank 7 through the pump 10 to the feed device 13. The direction from the tank 7 to the feed device 13 is designated as a feed direction 18.

Furthermore, the motor vehicle 11 has a control unit or controller 4. Active draining of the delivery line 9 of the delivery unit 1 can be carried out with the control unit 4. The control unit 4 is supplied with electrical energy and/or with electrical signals through an electrical connection 2 by a supply device or supplier 15. This electrical connection 2 has a switch 3 which is actuated as a function of temperature. When the temperature drops resulting in a (predefined, lower) threshold temperature being reached, the switch 3 closes the electrical connection 2 in order to activate the control unit 4. In addition, a deactivation switch 19 can also be provided on the electrical connection 2. The control unit 4 can control the deactivation switch 19 again later through a deactivation line 20 in order to deactivate the control unit 4 independently of the position of the switch 3. The control unit 4 can also have a memory 6 (which acquires and/or archives electrical data) in which information as to whether or not the delivery unit 1 has been drained is stored. The control unit 4 is connected to the pump 10 through a control line 16 in order to control the pump 10. It is furthermore advantageous if the switch 3 can be bypassed and/or blocked by a bypass 21 during operation of an internal combustion engine 8. The motor vehicle 11 preferably also has a temperature sensor 5 which is connected to the control unit 4 and with which the control unit 4 can independently monitor the temperature which is present, in order to check whether or not the draining of the delivery unit 1 is actually to be carried out.

FIG. 2 shows a switch 3 which is suitable for the described method and which is embodied as a bimetal switch 23. This bimetal switch 23 is composed of a first metal 24 and a second metal 25 which are permanently connected to one another. The first metal 24 and the second metal 25 have different coefficients of thermal expansion. The bimetal switch therefore deforms when the temperature changes. The bimetal switch 23 is configured in such a way that electrical contacts 22 make contact with one another when a threshold temperature is reached in order to produce an electrical connection 2.

FIG. 3 shows a switch 3 which is suitable for the described method and which functions by using a fluid (an expansion fluid 27). The switch 3 has a reservoir 28 which is filled with the expansion fluid 27. In order to ensure that the temperature of the surroundings effectively passes to the expansion fluid 27, the switch 3 also has a temperature pickup body 26 for picking up the ambient temperature and for passing the ambient temperature on to the expansion fluid 27. The expansion fluid 27 is connected to a piston 29. If the temperature of the expansion fluid 27 decreases or increases, the piston 29 moves. When a threshold temperature is reached, the piston 29 is moved in such a way that the electrical contacts 22 make contact with one another and an electrical connection is produced.

FIG. 4 shows a switch 3 which is suitable for the described method and is prestressed (mechanically). This switch 3 has an expansion material 35 which is, for example, a metal and which is lengthened or shortened as a function of the ambient temperature. The expansion material 35 is mounted in a frame 31 and is located in a first position 32 in which it is prestressed against a stop 30. The expansion material 35 which is illustrated in FIG. 4 preferably expands when the ambient temperature drops. When a threshold temperature is reached, the expansion material 35 has expanded to such an extent that it tips over or flips over from the first position 32 into a second position 33. An electrical connection is then produced between the two electrical contacts 22. An actuator 34, with which the expansion material 35 can be reset actively back into the first position 32, is also preferably provided. 

1. A method for draining a delivery unit for liquid additive, the method comprising the following steps: a) producing an electrical connection using a currentless switch constructed to produce an electrical connection when a temperature falls below a threshold temperature; and b) draining the delivery unit when, in step a), the electrical connection has been produced.
 2. The method according to claim 1, which further comprises activating a control unit by using the switch in step a), and controlling the draining in step b) by using the control unit.
 3. The method according to claim 2, which further comprises automatically deactivating the control unit after execution of the method.
 4. The method according to claim 1, which further comprises: performing a temperature measurement between step a) and step b) by using a temperature sensor in order to check if a threshold temperature has actually been undershot; and removing the liquid additive from the delivery unit in step b) only if the threshold temperature has actually been undershot.
 5. The method according to claim 1, which further comprises: storing the drainage of the delivery unit in a memory after step b); at least one of before step a) or after step a), checking if the delivery unit has already been drained based on information in the memory; and at least carrying out step b) only when the delivery unit has not been drained.
 6. The method according to claim 1, which further comprises carrying out the step of draining the delivery unit by back-suction of the liquid additive into a tank.
 7. The method according to claim 1, which further comprises selecting the switch at least from the following group: a bimetal switch, a switch including a shape memory alloy, a temperature switch with a liquid, and a prestressed switch.
 8. The method according to claim 1, which further comprises executing the method during a standstill phase of an internal combustion engine.
 9. A delivery unit for delivering a liquid additive, the delivery unit comprising: a tank; a delivery line connected to said tank; a pump configured to deliver liquid additive through said delivery line from said tank; and a currentlessly activatable switch connected to said pump and configured to produce an electrical connection when a threshold temperature is undershot.
 10. A motor vehicle, comprising: an internal combustion engine; an exhaust-gas treatment device configured to purify exhaust gases of said internal combustion engine; and a delivery unit according to claim 9 associated with said exhaust-gas treatment device. 